The present invention relates to the fabrication of ceramic sheets, particularly ceramic sheets suitable for the production of multi-layer ceramic devices.
Multi-layer ceramic (MLC) devices are fabricated from a plurality of stacked ceramic sheets, wherein each sheet has a particular screened metal pattern on its planar surface and a particular pattern of feed-through vias, or holes. The vias in the ceramic sheets are usually filled in the unfired or "green" state with a slurry of metal paste. Screen printing techniques, through either a stencil or a screen, are typically used to force the metallizing paste into the via holes. After the patterned and via-filled layers are stacked in the proper order, they are laminated together with heat and pressure to form an aligned assembly ready for firing.
During the firing operation, the binder is volatilized from the ceramic tape, the patterned metallization and the via metallization. After binder removal, the temperature is increased to provide densification by sintering of the ceramic and metal portion of the assembly. The metal filled via hole now becomes an electrical conductor and provides the desired electrical interconnection between the various layers of patterned metallization.
MLC devices may be composed of a number of different layers. These layers may include a top surface for integrated circuit (IC) attachment and wire bonding, redistribution or "fan out" layers, signal distribution layers, power distribution layers, and bottom surface pads.
Vias permit electrical interconnection between the multiple layers. Vias may also act as heat sinks for heat generating components placed on top of the device, or as electrical grounds for the components. The vias are most commonly formed using a tool to mechanically punch the holes into the ceramic layers.
The present trend is toward larger ceramic printed circuit boards with increased interconnect density, that is, an increased density of vias. For example, five inch square modules are now commonly in use. A five inch square module having vias on a 25 mil grid will require 40,000 via holes. A "mil" is equivalent to 0.001 inch, or about 0.0254 mm. As used herein, the phrase 25 mil grid refers to an array of via locations, wherein each via is located approximately 25 mils, or 0.025 inch, from each adjacent via in the same row. This description applies herein to any "grid" preceded by a measurement reference. Other parts may require a higher density, such as vias on a 10 mil grid. The distance between adjoining vias may also be referred to as the "pitch." For example, a 10 mil grid has a finer pitch than a 25 mil grid. While most designs will not require solid grid vias, it is not uncommon to manufacture a grid having, for example, 12,000 vias on one surface.
Typically, at least one of the ceramic layers in the device will have a via pattern that differs from the via pattern in one or more of the other ceramic layers. Hence, separate punch tools, known as permanent cluster punch tools, must be fabricated for each different layer. Often, the MLC device will have in excess of five layers, each having a different via pattern. This requires the fabrication of multiple punch tools, which are relatively expensive. The fabrication of a separate punch tool for each layer can be prohibitively expensive, particularly when the multi-layer devices are produced for a customer in relatively small quantities.
One alternative is the use of programmable punches, wherein each individual layer pattern is programmed into a computerized punch and the punch fabricates each layer individually. These numerically controlled (NC) punches are presently available from several suppliers. Typically, the programmable punch apparatus has a single punch which can be activated at a speed of about 10 punches per second. Thus, with this machine, one layer of a 12,000 via part will require about 20 minutes to punch. In comparison, a full permanent cluster punch tool can operate at punching rates between about 3 and 30 seconds per layer, independent of the number of vias in the layer.
Another alternative is the use of a laser drilling apparatus. Again, productivity rate is a severe limitation of laser drilling techniques. The best laser drilling apparatus known to the inventor at the present time is rated at about 40 vias per second. Other problems with laser drilling include poor via quality and excess material deposition around the edges of the vias.
There is a need for an inexpensive and effective process for the formation of via patterns in individual layers, so that the fabrication cost of MLC devices can be reduced. Such a process would be particularly useful for low quantity production devices and devices that have a large number of individual layers with differing via patterns. It would be particularly advantageous if the process could be carried out using equipment that manufacturers of multi-layer ceramic devices typically have in their possession.